Devlin was raised in California and joined the U.S. Army during World War II, rising to the rank of captain.

He entered service with the CIA in 1949, having been recruited out of college by McGeorge Bundy.

Devlin became chief of station in Congo in July 1960, a mere 10 days after the country's independence from Belgium and shortly before Prime Minister Patrice Lumumba's two month term in office, dismissal from power and ultimate execution. In his memoir, Devlin reveals that late in 1960, he received instructions from an agent ("Joe from Paris") who was relaying instructions from CIA headquarters that he (Devlin) was to effect the assassination of Lumumba. Various poisons, including one secreted in a tube of toothpaste, were proffered. The directive had come from the CIA Deputy Chief of Plans Dick Bissell, but Devlin wanted to know if it had originated at a higher level and if so, how high. "Joe" had been given to understand that it had come from President Dwight D. Eisenhower, but Devlin never knew for sure. Devlin wrote (and said in public speaking engagements) that he felt an assassination would have been "morally wrong" and likely to backfire and work against U.S. interests.[2] In the event, he temporized, neglecting to act, and Lumumba was ultimately murdered by his enemies in Katanga, allegedly with Belgian government participation. U.S. intelligence was kept apprised.

Devlin supported Congolese dictator Mobutu Sese Seko in his two coups (14 September 1960 and 24 November 1965). He maintained that it was not a mistake for the U.S. to support this anti-communist, pro-Western strongman in light of larger Cold War concerns.

Later, Devlin served as station chief in Laos, and then as chief, Africa Division. He retired from service with the CIA in 1974.

Subsequent to his CIA employment, Devlin settled with his wife in the Congo and became the business agent of Maurice Tempelsman who advised the Mobutu Government on its dealings with the De Beers diamond cartel in Kinshasa. Devlin also socialized widely in the expatriate community of Kinshasa during the late 1970s and early 1980s.[3]

In later life, Devlin divided his time between Virginia and Provence, France.

Devlin's book Chief of Station, Congo is his account of his firsthand experiences and observations in the Congo during the Cold War.[4]